Various forms of lamps, such as fluorescent and incandescent, include at least one fragile filament. Lamp filaments and other dynamic loads exhibit impedance that varies, for example, as a function of temperature (i.e., as the temperature of the filament increases due to current-induced heating, the impedance increases). When power is supplied to the lamp, the filament is usually cold and the resistance is low. At power-up, the initial current can be as high as ten to twenty times greater than the normal operating current. Repeated cold-current surges will degrade the filament and result in premature failure of the lamp.
The high initial current can be controlled with a soft-start circuit. Soft-start circuits are used to control the rate at which power is applied to the dynamic load. Generally, it is desirable to increase the power to the load in a smooth manner. Thus, controlling the rate of power application to the lamp results in heating the filaments at a slower rate and reduces the risk of filament damage.
One soft-start technique is a "trickle current" which provides a relatively small, continuous current to the dynamic load when it is not operating. The continuous flow of current keeps the load warm and the impedance high. When full power is suddenly applied, the surge current is reduced. The trickle current system, while simplistic, does require extraneous or sequenced power supplies and does not eliminate the surge current, only reduces it. Further, the continuous supply of current required to implement this technique can be costly and inefficient.
Another technique for reducing the surge current effect is through a thermistor or other temperature dependent resistance. When power is initially applied, the current flows through the thermistor producing rapid heating and high resistance. As the thermistor heats up, the resistance stabilizes and the operating current is achieved. A thermistor is rugged and relatively inexpensive, but its behavior is difficult to predict. A thermistor also dissipates a significant amount of power during normal operation which can affect its resistive values.
A series inductor may also limit surge current in some applications which require large current. Inductive chokes are magnetic components that obey Lenz's Law. At power-on, the magnetic field created by the inductor reduces the initial current and diminishes the sudden surge current to the load. In many environments, the addition of a strong magnetic field may not be desirable. Further, inductive chokes tend to be bulky, heavy and dissipate power during normal operation.
A current regulation system including a small sense resistor coupled to the load is yet another soft-start technique. The voltage across the resistor provides feedback for controlling the power supplied to the load. Such systems offer very brief control before full power-up, usually around 20 to 100 milliseconds, and this period may be too short for applications with large initial currents or particularly sensitive loads.